Photoacoustic imaging (PAI) is an emerging biomedical imaging modality based on the photoacoustic effect. In PAI, light pulses (often from a laser) are delivered to a target locus (“situs”) in or on a sample. Some of the pulse energy is absorbed at the situs and converted into heat. The transient heating causes a corresponding transient thermoelastic expansion of the situs, which produces a corresponding wideband ultrasonic emission from the situs. The generated ultrasonic waves are detected using one or more ultrasonic transducers that convert the detected waves into corresponding electrical pulses that are processed into corresponding images.
The optical absorption of light by a biological sample is closely associated with certain physiological properties such as hemoglobin concentration and/or oxygen saturation. As a result, the magnitude of ultrasonic emission (i.e., the photoacoustic signal, proportional to the local energy deposition) from the situs reveals physiologically specific optical absorption contrast that facilitates formation of 2-D or 3-D images of the situs. Blood usually exhibits greater absorption than surrounding tissues, providing sufficient endogenous contrast to allow PAI of blood vessels and tissues. For example, PAI can produce high-contrast images of breast tumors in situ due to the greater optical absorption by the increased blood supply provided to the tumor. While conventional X-ray mammography and ultrasonography produce images of benign features as well as pathological features, PAI can produce information more specific to the malignant condition, such as enhanced angiogenesis at a tumor site.
Another important imaging modality for various disease-related and other purposes is ultrasonic scanning, which has high sensitivity but low tissue specificity. Ultrasonic imaging is performed using an ultrasonic scanning machine to which a “probe” comprising one or more piezoelectric transducers is connected. Electrical pulses produced by the machine are converted by certain of the piezoelectric transducer(s) into corresponding ultrasound pulses of a desired frequency (usually between 1 and 70 MHz). The ultrasound is focused either by the shape of the sending transducer(s), a “lens” in front of the transducer, or a complex set of control pulses from the ultrasonic scanning machine. Focusing produces a shaped acoustic wave propagating from the transducer. The sound wave enters the sample (e.g., a subject's body) and converges to focus at a desired depth in the sample. Most ultrasonic probes include a face member made of a material providing impedance matching for transmitting ultrasonic pulses efficiently into the subject's body. A hydrogel is usually applied between the subject's skin and the face member of the probe for efficient propagation of sound waves to and from the probe. Portions of the sound wave are reflected from various tissue layers and structures in the sample, particularly from loci exhibiting density changes. Some of the reflected sound returns to the probe, in which certain transducer(s) convert the received sound into corresponding electrical pulses that are converted by the ultrasonic scanning machine into an image.
One of the challenges in photoacoustic (PA) imaging for in vivo animal study and eventual clinical translation is the limited penetration depth of the source light. Near infrared light allows PA imaging up to a few centimeters deep in soft tissues, but typically with unacceptable signal-to-noise ratio. Some applications require large depths that are limited by attenuation of source laser light propagating through soft tissues. A large portion of light energy directed into soft tissues is reflected and lost at the skin surfaces. In addition, irradiation with non-uniform optical beams can produce measurement artifacts that are more closely related to optical beam non-uniformity rather than specimen features. A non-uniformly diffused laser beam to the skin with high power may also cause damage such as burning.